The present invention relates, in general, to materials handling and, in particular, to feeding apparatus which delivers controlled amounts of materials to utilization apparatus.
FIG. 1 is a schematic drawing of a typical prior art feeding system of the type to which the present invention relates. Material, for example, in the form of pellets is drawn from a silo 10 through an inlet tube 12 to a loader 14, such as the hopper which is shown in FIG. 1 or onto a conveyor, by a vacuum pump 16 acting through a vacuum tube 18. Hopper loader 14 discharges controlled amounts of the material to a refill hopper 20, below the hopper loader, through a flap valve 22. The material in refill hopper 20, in turn, is discharged to a feeder hopper 24, positioned below the refill hopper, through an outlet valve 26 at the bottom of the refill hopper. The material in feeder hopper 24, in turn, is supplied through an opening 27 at the bottom of the refill hopper to a feed unit 28, in this example a feed screw unit, which is positioned below the feeder hopper and, in turn, delivers controlled amounts of the material to utilization apparatus (not shown). The utilization apparatus can be, for example, extrusion equipment.
Not shown in FIG. 1 are the means by which the various components of the feeding system are controlled to transfer proper amounts of the material at the proper times. Flap valve 22 is controlled by the level of material in refill hopper 20. Flap valve 22 remains open, after material is discharged from hopper loader 14, while material remains in the hopper loader and closes when outlet valve 26 is opened. Outlet valve 26 is controlled by the level of material in feeder hopper 24. Vacuum pump 16 is controlled by flap valve 22 and serves to draw material from silo 10 through inlet tube 12 to hopper loader 14.
A typical application of the feeding system shown in FIG. 1 is in a blender where two or more such systems are combined to deliver controlled amounts of two or more constituent parts of a blend to a collection hopper which, in turn, delivers the blend to utilization apparatus. In such an application, the feed unit, in this example a feed screw unit, of each feeding system is operated both in a "gravimetric" mode and in a "volumetric" mode. In the gravimetric mode, the speed of the feed screw unit of the feeding system is controlled by weight changes in the feeder hopper caused by changes in the amount of the material in the feeder hopper, so that, over time, a varying amount of material is delivered by the feed screw unit to the collection hopper. In the volumetric mode, the speed of the feed screw unit is held constant, so that, over time, a constant amount of material is delivered by the feed screw unit to the collection hopper. Each feeding system is operated in the gravimetric mode, except during the times that the feeder hopper is being loaded with material from the refill hopper, at which times the feeding system is operated in the volumetric mode. Thus, material is delivered to the collection hopper by the feed screw unit even while the feeder hopper is being loaded, but at a constant rate, rather than at a varying rate, because the speed of the feed screw unit is held fixed. A typical time division, over a one minute full cycle, between operation in the gravimetric mode and operation in the volumetric mode, for a feeding system having, for example, a twenty liter hopper loader, a fifty liter refill hopper, and a fifty liter feeder hopper, is forty seconds of operation in the gravimetric mode and twenty seconds of operation in the volumetric mode.
Feeding systems of the type shown in FIG. 1 have certain shortcomings. By using three distinct and separate containers for the material (i.e. hopper loader 14, refill hopper 20, and feeder hopper 24) in the system for delivery of material from silo 10 to feed screw unit 28, the overall height of the apparatus is undesirably large. In addition, the controls for effecting proper delivery of material from one container to another, in terms of both the amount of material and the timing of the delivery, involve more components and complexity than are desired.
When two or more such systems are combined in a blender application, it is common practice to use a single vacuum system to fill the hopper loaders. The vacuum system is switched between the feeding systems in a predetermined sequence to deliver the constituent parts of the blend to the individual hopper loaders. Such operation limits the flexibility of the blender unless the hopper loaders are sized to accommodate the delivery of material when, in fact, material is not required. Additionally, if access to the vacuum source is provided to the feeders in a fixed sequence, a feeder having a high mass feed rate could potentially become empty before it obtains access to the vacuum source. Of course, an alternative is to have two or more separately controlled vacuum systems, for example one for each feeding system, which respond more closely to the individual demands for material of the individual feeding systems. However, such vacuum systems are expensive.